Fix for elements with no style attribute

I've got the following error:
```
TypeError: Array.prototype.forEach called on null or undefined
    at forEach (native)
    at exports.createWindow.DOMWindow.getComputedStyle (/Users/Stefan/Developer/Projects/Github/fabric.js/node_modules/jsdom/lib/jsdom/browser/index.js:279:15)
```

This is the code to initialize document and window elements under node:
````
fabric.document = require("jsdom").jsdom("<!DOCTYPE html><html><head></head><body></body></html>");
fabric.window = fabric.document.createWindow();
```
If i run this code i've got the above error:
```
var element = fabric.document.createElement('canvas');
var style = fabric.document.defaultView.getComputedStyle(element, null);
```
In the browser does this work.